docs: def assert_true(expr, msg_fmt="{msg}"):
      """Fail the test unless the expression is truthy.
  
      >>> assert_true("Hello World!")
      >>> assert_true("")
      Traceback (most recent call last):
          ...
      AssertionError: '' is not truthy
  
      The following msg_fmt arguments are supported:
      * msg - the default error message
      * expr - tested expression
      """
  
      if not expr:
          msg = "{!r} is not truthy".format(expr)
          fail(msg_fmt.format(msg=msg, expr=expr))